A–13–year old boy is referred for evaluation of nocturnal enuresis and short stature. His blood pressure is normal. The hemoglobin level is 8g/dl.urea 112 mg/dl, creatinine 6 mg/dl, sodium 119 mEq/ dl, potassium 4 mEq/l, calcium 7 mg/dl, phosphate 6 mg/dl and alkaline phosphatase 300 U/I. Urinalysis shows trace proteinuria with hyaline casts; no red and white cells are seen. Ultrasound shows bilateral small kidneys and the micturating cystourethrogram is normal. The most likely diagnosis is –